Most electronic and computer systems require DC voltage for their operation. A typical power supply for such systems converts AC line current to a regulated DC voltage. A "switch mode" type power supply first converts AC to an unregulated DC which is then controllably pulsed to a regulated DC output. For example, conversion of AC to an unregulated DC may be achieved by rectifying the AC and storing the nonregulated DC voltage in bulk capacitors. Current from the bulk capacitors is then periodically switched or pulsed across a primary winding of a transformer. The resulting square wave is received by a secondary winding of the transformer, rectified, regulated, and stored in capacitors which provide the primary DC output voltage of the power supply.
The means used for switching current from the bulk capacitors may be transistors controlled by a series of pulses. These pulses are provided by a pulse width modulator (PWM) chip. In order to generate pulses, the PWM requires a predetermined input voltage. Some power supplies are designed so that during operation the PWM input voltage is provided by an auxiliary voltage obtained from a secondary winding or primary winding output of the transformer. However, in order to power the PWM chip when power is not available from the transformer, some type of start-up circuit is required. Some prior art start-up circuits consist of a circuit connecting the bulk capacitors to the PWM input. One problem with such start-up circuits is that they continue to drain current after the power supply is up and running and obtaining PWM input voltage from the transformer.
Other problems with start-up circuits relate to the fact that they are not independent of other components of the power supply. For example, power supplies for equipment such as a printer may receive a relatively large initial power demand when the printer is first turned on. Such power demands may prevent the primary winding output of the transformer from providing the auxiliary voltage needed for the PWM. However, because of the large demand for power, it may be undesirable to continue to drain current through the start-up circuit during this transitory, high demand period.
A further problem for start-up circuits may be caused by an abnormal condition on the output such as a short circuit or overload. The unusually large power drain may again reduce the auxiliary voltage from the level needed to power the PWM. This will increase the current flow through the start-up circuit as it attempts to provide the needed power. The longer or more frequent the demand for power from the start-up circuit, the larger the start-up circuit elements need to be. Larger elements require more layout space and generally add to the expense of power supply production.
Some power supplies are designed to switch off immediately when an abnormal condition occurs on the output. For those power supplies which do not immediately switch off, an abnormal condition on the output will not only increase demands on the start-up circuit, but the bulk capacitors too will draw increased current. The bulk capacitor charging current at power supply turn on is sometimes referred to as the in-rush current. Limiting the in-rush current will prevent overheating. When the in-rush current is limited there is no need to continue to power the PWM through the start-up circuit since the bulk capacitors are not being charged.